Report of the Benchmark Workshop on Pelagic Stocks (WKPELA). 17–21 February 2014 Copenhagen, Denmark

WKPELA meeting was held in ICES HQ in Copenhagen from the 21–27 February 2014, to benchmark the assessments of herring in the Celtic Sea and mackerel in the Northeast Atlantic. The data compilation process and intercessional work began in October 2013. The assessment of both of these stocks was previously done using a statistical catch-at-age model ICA, which imposed structural assumptions on the data; the validity of some of these assumptions had been questioned in recent years. In the case of NEA mackerel the previous assessment was not considered to give a reliable estimate of the development of the stock, and this assessment was limited by the lack of independent age-structured indices. New data which was examined for mackerel included fishery-independent data (acoustic surveys, bottom-trawl surveys, and a swept-area trawl survey), as well as a re-examination of the tagging data, landings and discard and biological data. For NEA mackerel the benchmark workshop agreed updates to the input data on catch, weight-at-age, maturity ogive, and changes to the estimation methods for other assessment parameters such as proportion of F and M before spawning. There was also an agreement to include age-structured indices on adults (from the IESSNS swept-area trawl survey) and recruits (IBTS Q4 trawl survey), and to use the tagging data as an index of population abundance-at-age using a Petersen estimator. The IESSNS survey was proposed to be included in the assessment as an abundance index at age, although it was also suggested as a research need, to investigate its use as an index of relative proportions at age only (i.e. without the trend in abundance). The tagging index was considered to give useful information on population abundance at up to 2007, around which time the methodologies changed and the recapture rates dropped inexplicably. There is a research recommendation to investigate the changes in the tagging methodologies (both release and recapture), with a view to updating the tagging dataseries when the changes in observed recaptures has been resolved. Two assessment models were explored, and one (SAM) was put forward as the method to be applied in the stock annex. The SAM model differs from the previous NEA mackerel assessment model in that it is a random effects model. This means that parameters such as for e.g. fishing mortality and initial population numbers are assumed to be correlated over time and are estimated using a random walk function, rather than by fitting to a fixed effect. The model is flexible and can cope with high levels of variance in the observation data, giving it an appropriate application in fisheries stock assessment. The benchmarked assessment for NEA mackerel changes the perception of the development of the stock over time, whereby the biomass both in the early period (1980s) and more recently is higher than previously estimated, and F lower. Reference points were re-examined and the new perception of the stock resulted in a slight revision of these to a slightly higher Blim (and thus BPA), and commensurately Flim and FPA. FMSY is now estimated to be slightly higher also and this is expected to lead to a revision of the management plan, notwithstanding the fact that the existing exploitation range in the management plan could be considered consistent with the PA. In the case of Herring in the Celtic Sea, the input data remains the catch and biological data and the HERAS survey. There was a change to the natural mortality rate assumed, which brings this stock more in line with others in the European shelf area. The age range used from the survey and in the assessment was also revised as an examination of the interpretation of the data by the assessment model showed that there was valuable signal in the youngest ages on the survey (1 ringer) and in the older ages in the catch (extension of the plus group from 6+ to 9+). Large changes in mean weights were observed for this stock and are expected to have an effect on reference points, however due to time constraints reference points for herring were not proposed by WKPELA, and it this work was to be undertaken in March 2014 by the HAWG (herring assessment working group). Stock annexes for mackerel in the NEA Atlantic and herring in the Celtic sea were completed according to the work done.